1. Field of the Invention
The present invention relates to a method of fabricating a thin oxide layer on a silicon substrate, more particularly, to a method of forming an ultra-thin oxide layer on a silicon substrate to be used as a gate oxide for a metal oxide semiconductor (MOS) device.
2. Description of the Related Art
In very large scale integration (VLSI) devices and in particular in metal oxide semiconductor (MOS) devices, as the length of the gate electrode and the thickness of the gate oxide are decreased, the MOS device can be switched at higher speed. However, decreases in the thickness of the gate oxide can result in larger amount of leakage current flow through the gate oxide, and an excessive amount of standby power may be dissipated. The leakage current varies exponentially with the thickness of the gate oxide.
One way to reduce the leakage current is to incorporate a high concentration of nitrogen into the gate oxide. The nitrogen in the gate oxide increases the dielectric constant of the gate oxide. Owing to the increase in the dielectric constant of the gate oxide, a film (e.g., oxynitride layer) thicker than a pure silicon oxide layer may be used as the gate oxide while maintaining an equivalent capacitive value. Thus, the leakage current in the gate oxide can be reduced by using the thicker film.
One way to introduce nitrogen into the gate oxide is to implant a high dose of nitrogen into a silicon substrate and subsequently grow the gate oxide on the nitrogen implanted silicon substrate.
However, as nitrogen dosage is increased above a certain amount, the thickness of the gate oxide is also increased. The threshold nitrogen dose is in a range from 1e15 cmxe2x88x922 to 4e15 cmxe2x88x922.
Accordingly, a need exists for a method of reducing leakage current in MOS devices using ultra thin gate oxides.
It is an object of the present invention to provide a method for forming an ultra-thin gate oxide on a silicon substrate for MOS devices where nitrogen is implanted in the silicon substrate.
It is another object of the present invention to provide a method for forming an ultra-thin oxide layer on a silicon substrate where the nitrogen implanted substrate is annealed so that the thickness of the oxide layer decreases.
It is still another object of the present invention to provide a method for forming an ultra-thin oxide layer on a silicon substrate where the nitrogen implanted substrate is annealed so that an amorphous layer formed due to the nitrogen implantation is recrystallized, wherein most of the implant damage is annealed out and most of the implanted nitrogen is still in the silicon substrate after the anneal.
To achieve the above and other objects, a method according to the present invention is provided for forming an oxide layer on a silicon substrate, comprising the steps of forming a sacrificial oxide layer on the silicon substrate, implanting nitrogen into the silicon substrate, annealing the silicon substrate having implanted nitrogen, removing the sacrificial oxide layer from the silicon substrate, and forming an oxide layer on the silicon substrate. The annealing step is preferably performed at temperatures in a range from about 550xc2x0 C. to about 1000xc2x0 C., more preferably, in a range from about 650xc2x0 C. to about 900xc2x0 C., and for a time period between about 1 second and about 2 hours, more preferably, for a time period between about 5 seconds and about 30 minutes. The annealing step is preferably performed in an inert gas ambient, more preferably, in a nitrogen or an oxygen ambient.
According to another aspect of the present invention, there is provided a method for forming an oxynitride layer on a silicon substrate, comprising the steps of implanting nitrogen into the silicon substrate, the nitrogen having a dosage above about 1e15 cmxe2x88x922, annealing the silicon substrate at temperatures between about 550xc2x0 C. and about 1000xc2x0 C. for a time period between about 1 second and about 2 hours, wherein an amorphous layer in the silicon substrate created during nitrogen implantation is recrystallized and most of the implant damage is annealed out, and forming an oxynitride layer through oxidation of silicon. The annealing thermal budget should be chosen such that most of the nitrogen will not diffuse to the oxide/silicon interface.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.